1. Field of the Invention
The present invention relates to vertical resolution correcting circuits and, more particularly, to a circuit for correcting vertical resolution of each of a luminance signal and a chroma signal which constitute a video signal.
2. Description of the Background Art
Conventionally, various techniques for correcting vertical resolution as well as reducing noise components of each of a luminance signal and a chroma signal which constitute a video signal are adopted for the purpose of improving picture quality in video recording and reproduction apparatus such as a video cassette recorder (hereinafter referred to as a VCR).
FIG. 1 is a schematic block diagram illustrating a structure of a reproduction system in a conventional VCR using such a vertical resolution correction technique as well as a noise reducing technique. Referring to FIG. 1, a video signal reproduced from a magnetic tape 1 by a reproduction head 2 is amplified by a pre-amplifier 3 and then applied to a high-pass filter (HPF) 4 and to a low-pass filter (LPF) 5.
A luminance signal (Y signal) is extracted from the reproduced video signal by high-pass filter 4 and applied to a luminance signal processing circuit 6. Luminance signal processing circuit 6 includes a double limiter, an FM demodulation circuit, a de-emphasis circuit, a nonlinear de-emphasis circuit, and the like, which are well known, and detailed description of them will be omitted.
Predetermined signal processing is performed on the luminance signal Y by luminance signal processing circuit 6, and the reproduced luminance signal Y is then applied to a vertical resolution correction circuit 7 and has the vertical resolution corrected. FIG. 2 is a block diagram illustrating details of a vertical resolution correcting circuit 7. Such a vertical resolution correcting circuit is disclosed, for example, in Japanese Patent Laying-Open No. 2-15784 (1990).
Referring to FIG. 2, reproduced luminance signal Y, which has been applied from luminance signal processing circuit 6 in FIG. 1 through an input terminal 20 to vertical resolution correcting circuit 7, is applied to an NH delay line (n is a positive integer, 1 in this embodiment, and H indicates a horizontal synchronization period of a video signal) 21, to a positive input of a subtracter 22, and to one input of an adder 26. Reproduced luminance signal YD, which has been delayed by a period of 1H by 1H delay line 21, is then applied to a negative input of subtracter 22 which, and subtracter 22 provides the difference between the present reproduced luminance signal Y and the reproduced luminance signal YD of 1H period before, i.e. the difference between luminance signals in two adjacent horizontal lines, as a noncorrelation luminance signal component .vertline.Y-YD.vertline. in the vertical direction. The noncorrelation signal component is applied to limiter 23 and to a positive input of a subtracter 24, while it is also applied through an output terminal 28 to vertical resolution correcting circuit 14 as illustrated in FIG. 1 to be used for vertical resolution correction of a chroma signal which will be described later. Detailed description of vertical resolution correction circuit 14 will be given later.
Limiter 23, which has a limiter level of 10.multidot.IRE, for example, passes an input having a level within the range of 0.multidot.IRE to 10.multidot.IRE as it is, and, when the level of the input is 10.multidot.IRE or more, provides a fixed output of 10.multidot.IRE. IRE is a unit expressing a level of a video signal, which expresses levels of a video signal ranging from a pedestal level to a white level of 0 to 100. The output of limiter 23 is applied to a negative input of subtracter 24. Specifically, subtracter 24 subtracts the output of limiter 23 from the input of limiter 23, so that the output from subtracter 24 is 0 when the abovedescribed noncorrelation signal component provided from subtracter 22 is at a level within the range of 0.multidot.IRE to 10.multidot.IRE, and, when the level of the noncorrelation signal component is 10.multidot.IRE or more, a signal at a level obtained by subtracting 10.multidot.IRE from the level of the noncorrelation signal component is provided. The output of subtracter 24 is multiplied by K in a K multiplying circuit 25, then applied to another input of adder 26, and added to reproduced luminance signal Y supplied from terminal 20.
FIG. 3 is a graph showing the relation between the level of the noncorrelation signal component (indicated on the abscissa) and the output of K multiplying circuit 25 (indicated on the ordinate), i.e. a characteristic of emphasis on the level of the luminance signal. Referring to FIG. 3, in view of the fact that the noncorrelation signal component .vertline.Y-YD.vertline. usually consists of only noise components in the range of 0.multidot.IRE to 10.multidot.IRE, the output of K multiplying circuit 25 is at 0 level in this range. Specifically, when the noncorrelation signal component is within the range of 0.multidot.IRE to 10.multidot.IRE, reproduced luminance signal Y is provided without emphases through a terminal 27. On the other hand, when the level of the noncorrelation signal component .vertline.Y-YD.vertline. exceeds 10.multidot.IRE, it is considered that this component is almost the original noncorrelation component of the luminance signal, so that a signal which increases according to the increase in the level of the noncorrelation component is added as an emphasis component to reproduced luminance signal Y.
As a result, in a region where the level of the noncorrelation signal component is lower than 10.multidot.IRE (a region where changes in the vertical direction of the luminance signal are even), it does not happen that the noise component is emphasized and added to reproduced luminance signal Y. However, in a region where the level of the noncorrelation signal component is 10.multidot.IRE or more, the noncorrelation signal component is added to the reproduced luminance signal, so that a contour in the vertical direction is emphasized, and resolution of the image as a whole is enhanced.
The reproduced luminance signal on which such contour emphasis processing has been performed is supplied through terminal 27 to a high-pass noise canceler (HPNC) 8 provided in a subsequent stage for reducing noise, cross modulation and beat. An output of HPNC 8 is applied to one input of an adder 9 for Y/C mixing.
A low frequency band converted chroma signal (C signal) is extracted from the reproduced video signal by low-pass filter 5 and applied to an automatic chroma controlling circuit (ACC) 10. The low frequency band converted chroma signal, which has its signal level made constant by ACC 10, has its frequency converted from 629 kHz to 3.58 MHz by a frequency converting circuit 11, to be applied through a band-pass filter (BPF) 12 to a comb filter 13. Comb filter 13 is of a well-known structure, for removing a crosstalk signal generated between adjacent tracks on a magnetic tape. An output of comb filter 13 is applied to a vertical resolution correcting circuit 14 and has the resolution in the vertical direction corrected. FIG. 4 is a block diagram illustrating details of vertical resolution correcting circuit 14. Such a vertical resolution correcting circuit is disclosed, for example, in U.S. Pat. No. 4,443,817.
Referring to FIG. 4, chroma signal C, which has been applied from comb filter 13 in FIG. 1 through an input terminal 30, is applied to one input terminal 31a of a proportional switch 31 and applied to a positive input of a subtracter 32. An output of subtracter 32 is applied to another input terminal 31b of proportional switch 31. Proportional switch 31 synthesizes the signal levels of input terminals 31a, 31b with a ratio of k.sub.1 :k.sub.2 (K.sub.1 +k.sub.2 =1) changing according to a control signal applied to a control input terminal 31c and provides the obtained signal level through an output terminal 31d. The output from the output terminal 31d is applied to a 1H delay line 34 and to one input of an adder 35, while it is also provided as a chroma signal on which vertical resolution correction processing has been performed and applied through an output terminal 42 to adder 9 (FIG. 1) for Y/C mixing in a subsequent stage.
Chroma signal CD, which has been delayed by a period of in 1H delay line 34, is applied to another input of adder 35 and also to an n multiplying circuit (n is 0.7, for example) 33. Chroma signal CD, which has been multiplied by n in n multiplying circuit 33, is applied to a negative input of subtracter 32. As described above, signal C is applied to one input terminal 31a of proportional switch 31, and C-nCD, an output of subtracter 32, is applied to another input terminal 31b, so that the signal level at output terminal 31d is expressed as k.sub.1 C+k.sub.2 (C-nCD).
Accordingly, the noise amount to be removed is increased as the value of k.sub.2 is increased. In such a case, however, the ratio of CD is relatively increased, resulting in the occurrence of chromatic blur.
The above ratio of k.sub.1 :k.sub.2 is changed according to the signal level at control input terminal 31c on the basis of the presence or absence of correlation of a video signal in the vertical direction for the purpose of preventing chromatic blur in the vertical direction in the boundary between colors on a reproduced image. Specifically, in a case where it is determined that a chroma signal has correlation in the vertical direction, the condition of k=0 and k.sub.2 =1 is realized, and an output of proportional switch 31, i.e. an output of vertical resolution correcting circuit 14, is C-nCD. On the other hand, in a case where it is determined that there is no correlation, the condition of k.sub.1 =1 and k.sub.2 =0 is realized, and the output is C. In an intermediate case between both the extreme cases, for example, in a case where the condition of k.sub.1 =k.sub.2 =0.5 is realized, the output is 0.5C+0.5 (C-nCD).
Such determination of presence or absence of correlation in the vertical direction is performed by detecting correlation of a chroma signal and correlation of a luminance signal. Specifically, referring to FIG. 4, a noncorrelation signal component of the luminance signal supplied from output terminal 28 of vertical resolution correcting circuit 7 in FIG. 2 is applied through a terminal 38 and a LPF 39 to a rectangular wave correlation detecting circuit 40 which provides a detection output when the noncorrelation signal component of the luminance signal exceeds a predetermined threshold value. The output of rectangular wave correlation detecting circuit 40 is applied to one input of an adder 41. A noncorrelation signal component of the chroma signal provided from an adder 35 is applied through BPF 36 to a rectangular wave correlation detecting circuit 37 which provides a detection output when the noncorrelation signal component of the chroma signal exceeds a predetermined threshold value. The output of rectangular wave correlation detecting circuit 37 is applied to another input of adder 41, and an output of adder 41 is applied to a control input terminal 31c of proportional switch 31.
Specifically, in this conventional technique, generation of chromatic blur in a reproduced image is prevented by changing the ratio of synthesis in proportional switch 31 according to the degree of correlation in the vertical direction in both of a luminance signal and a chroma signal on the assumption that the degree of vertical correlation of the luminance signal has relatively strong relevance to the degree of vertical correlation of the chroma signal.
The chroma signal on which such chromatic blur preventing processing has been performed is applied through a terminal 42 to another input of adder 9 (FIG. 1) for Y/C mixing. Then, a reproduced video signal on which various vertical resolution correcting processing has been performed is provided from adder 9.
The conventional vertical resolution correcting circuit 7 illustrated in FIG. 2 has a problem as described in the following. In circuit 7 in FIG. 2, if the noncorrelation signal component of a luminance signal exceeds 10.multidot.IRE, an amount of emphasis which increases according to the noncorrelation signal component increase is added to a reproduced luminance signal as described with reference to FIG. 3, and such addition is also performed in a case where the noncorrelation component exceeds 40.multidot.IRE as shown in FIG. 3.
FIG. 5 is a partial enlarged diagram of a vertical blanking period of a video signal, wherein each of the synchronizing pulses and equalizing pulses has an amplitude of 40.multidot.IRE. Accordingly, when a noncorrelation component is detected with respect to an interval of 1H period as indicated by A in a discontinuous part in the boundary between an equalizing period and a video period, the amplitude of the equalizing pulse, 40.multidot.IRE, is erroneously detected as the noncorrelation component of the luminance signal, and an amount of emphasis corresponding to 40.multidot.IRE is added unnecessarily to the video signal as indicated by a part B depicted with oblique lines in FIG. 3, so that the picture quality of a reproduced image is degraded.
The conventional vertical resolution correcting circuit 14 illustrated in FIG. 4 has a problem as described in the following. According to the conventional example illustrated in FIG. 4, an output of circuit 14 is controlled on the assumption that the degree of vertical correlation of a luminance signal has relevance to the degree of vertical correlation of a chroma signal. However, in an actually reproduced video signal, there is a case where only a chroma signal changes largely although a luminance signal changes little, and there is also a case converse to it, for example. Accordingly, in such cases, there is a problem that it is impossible to perform vertical resolution correction operation on the basis of the degree of correlation of a chroma signal in circuit 14 in FIG. 4.
In addition, circuit 14 in FIG. 4 has a problem that the circuit configuration is complicated because the ratio of the proportional switch is changed on the basis of correlation detecting results of both of a luminance signal and a chroma signal.